U.S. patent number 6,842,754 [Application Number 09/836,828] was granted by the patent office on 2005-01-11 for lease enforcement in a distributed file system.
This patent grant is currently assigned to Hewlett Packard Development Company, L.P.. Invention is credited to Daniel A. Muntz.
United States Patent |
6,842,754 |
Muntz |
January 11, 2005 |
Lease enforcement in a distributed file system
Abstract
System and method for managing leases in a distributed file
system. A meta-data server is employed to manage leases to the
objects in the distributed file system, and a plurality of storage
servers provide data storage for the objects. The meta-data server
grants leases for objects to the clients. With each lease granted,
the meta-data server provides data that indicate the time at which
the lease will expire for the associated object. The lease
expiration time is included in a subsequent request to the storage
server for access to the object. The storage server determines
whether the lease has expired. If the lease has expired, the
requested access is denied. Otherwise, the storage server provides
the requested access.
Inventors: |
Muntz; Daniel A. (Cupertino,
CA) |
Assignee: |
Hewlett Packard Development
Company, L.P. (Houston, TX)
|
Family
ID: |
25272831 |
Appl.
No.: |
09/836,828 |
Filed: |
April 17, 2001 |
Current U.S.
Class: |
1/1; 707/E17.01;
707/999.2; 707/999.01 |
Current CPC
Class: |
G06F
16/10 (20190101) |
Current International
Class: |
G06F
17/30 (20060101); G06F 7/00 (20060101); G06F
017/30 () |
Field of
Search: |
;707/1-10,100-104.1,200-206 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Cary G. Gray et al., Leases: An Efficient Fault-Tolerant Mechanism
for Distributed File Cache Consistency, ACM, 202-210.* .
Liskov , "Practical Uses of Synchronized Clocks in Distributed
Systems", ACM, 1991, 1-9..
|
Primary Examiner: Alam; Shahid
Assistant Examiner: Fleurantin; Jean Bolte
Claims
What is claimed is:
1. A computer-implemented method for managing access to objects by
clients in a distributed file system including a storage server
arrangement and a meta-data server, comprising: managing leases on
the objects at the meta-data server; transmitting lease expiration
data from the clients to the storage server arrangement along with
storage access requests, the lease expiration data indicating a
lease expiration time; comparing at the storage server arrangement
the lease expiration data to data indicating a current time; and
denying access to the object if the lease expiration time is
earlier than a current time.
2. The method of claim 1, further comprising transmitting lease
requests from the clients to the meta-data server, each lease
request including an object identifier and a requested lease
duration.
3. The method of claim 2, further comprising for each lease
granted, returning data to a requesting client indicating a time at
which the lease began and a duration of the lease.
4. The method of claim 3, further comprising computing lease
expiration times at the clients in response to leases granted,
wherein the lease expiration data specify the lease expiration
times.
5. The method of claim 1, further comprising computing lease
expiration times at the meta-data server, and transmitting data
indicating the lease expiration times from the meta-data server to
the clients.
6. A computer-implemented method for managing access to file data
in a distributed file system including a storage server
arrangement, a meta-data server, and a plurality of clients
comprising: submitting a lease request from a client to the
meta-data server, the lease request referencing an object in the
distributed file system; when the object becomes available for
lease, designating the object as leased to the client and
transmitting a lease response to the client, the lease response
including data that indicate a lease expiration time; transmitting
a storage access request referencing the object from the client to
the storage server arrangement, the storage access request
including data that indicate the lease expiration time; and denying
access to the object if the lease expiration time is earlier than a
current time.
7. The method of claim 6, wherein the lease request includes an
object identifier and a requested lease duration.
8. The method of claim 7, wherein the data indicating the lease
expiration time describes a time at which the lease began and a
duration of the lease.
9. The method of claim 8, further comprising computing the lease
expiration time at the client from the lease response.
10. The method of claim 6, further comprising computing the lease
expiration time at the meta-data server.
11. An apparatus for managing access to objects by clients in a
distributed file system including a storage server arrangement and
a meta-data server, comprising: means for managing leases on the
objects; means for communicating lease expiration data from the
clients to the storage server arrangement along with storage access
requests, the lease expiration data indicating a lease expiration
time; and means for conditionally providing access to the object at
the storage server arrangement if the lease expiration time is
later than a current time.
12. A system for managing access to objects by clients in a
distributed file system, comprising: a meta-data server coupled to
the clients, the meta-data server configured and arranged to manage
leases to the objects responsive to requests from the clients, and
transmit lease expiration data to the clients indicating lease
expiration times; and a storage server arrangement coupled to the
clients, the storage server arrangement configured and arranged to
conditionally provide access to the objects in response to access
requests from the clients that include data indicating lease
expiration times, wherein access is provided in response to a
request if the lease expiration time is earlier than a current
time.
13. The system of claim 12, wherein the lease request includes an
object identifier and a requested lease duration.
14. The system of claim 13, wherein the data indicating the lease
expiration time describes a time at which the lease began and a
duration of the lease.
15. The system of claim 14, wherein the meta-data server is further
configured to compute lease expiration times.
Description
FIELD OF THE INVENTION
The present invention generally relates to distributed file
systems, and more particularly to lease enforcement in distributed
file systems.
BACKGROUND
Conceptually, a physical file system architecture includes system
meta-data, M.sub.sys, object meta-data, M.sub.object, and object
data. The M.sub.sys data describes the entire file system. For
example, M.sub.sys, includes system parameters, an i-node bitmap, a
block bitmap and other file system-dependent information.
The M.sub.object data describes a file or directory. In an example
physical file system, a particular M.sub.object segment includes
the file i-node and direct and indirect blocks. The i-node includes
information such as the name of the file, access rights, update
times, pointers to data blocks, and pointers to indirect, double
indirect, and triple indirect blocks. The indirect blocks also
contain pointers to data blocks. The file data is the data
referenced and manipulated by one or more client application
programs.
A distributed file system is generally characterized by multiple
storage servers that store and serve data to client applications.
The storage servers and systems that host the client applications
are connected to a network. A locking mechanism provides data
consistency as between multiple clients. For example, when a client
is writing to a file, the i-node is locked to prevent another
client from manipulating the same file. Any operations that result
in allocating or de-allocating i-node or data blocks cause the
associated bitmap areas to be locked.
In some distributed file systems, the locking mechanism is provided
by a meta-data server. The meta-data server manages some
implementation-dependent combination of M.sub.sys and M.sub.object,
and separate storage servers provide storage for file data. A lease
mechanism is often used for locking objects. Leases are managed and
granted by the meta-data server. A lease for an object is for a
selected period of time and is granted in response to a lease
request from a client. The type of lease granted to one client
determines whether a lease will be granted to another client. For
example, if one client is granted a write lease, no other client
will be granted a write lease until the first write lease
expires.
The separation of storage servers from the meta-data server creates
the potential for timing-related problems in servicing write
leases. Since the storage server is not involved in the management
of leases, it does not have the information necessary to enforce
data consistency by providing or denying file access requested from
a client. However, if too much information is shared between the
meta-data server and the storage servers, the benefits of
scalability provided by the separation of functions would be
adversely impacted.
A system and method that address the aforementioned problems, as
well as other related problems, are therefore desirable.
SUMMARY OF THE INVENTION
In various embodiments, the invention manages leases in a
distributed file system. A meta-data server is employed to manage
leases to the objects in the distributed file system, and a
plurality of storage servers provide data storage for the objects.
The meta-data server grants leases for objects to the clients. With
each lease granted, the meta-data server provides data that
indicate the time at which the lease will expire for the associated
object. The lease expiration time is included in a subsequent
request to the storage server for access to the object. The storage
server determines whether the lease has expired. If the lease has
expired, the requested access is denied. Otherwise, the storage
server provides the requested access.
It will be appreciated that various other embodiments are set forth
in the Detailed Description and Claims which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects and advantages of the invention will become
apparent upon review of the following detailed description and upon
reference to the drawings in which:
FIG. 1 is a functional block diagram of a computing arrangement in
accordance with one embodiment of the invention;
FIG. 2 is an object interaction diagram that illustrates two
clients seeking write access to the same object;
FIG. 3 is a flowchart of a process performed by a client in
submitting a lease request and a storage access request in
accordance with one embodiment of the invention;
FIG. 4 is a flowchart of a process performed by the meta-data
server in processing a lease request in accordance with one
embodiment of the invention; and
FIG. 5 is a flowchart of a process performed by the storage server
in processing a storage access request in accordance with one
embodiment of the invention.
DETAILED DESCRIPTION
The present invention is directed to lease management in a
distributed file system, where a meta-data server manages the file
system meta-data and a plurality of storage servers manage file
data. The meta-data server manages the granting of leases to
clients, each lease having an expiration time. After a client has
been granted a lease, the client generates a storage access request
that includes a reference to an object and in addition, the
expiration time of the lease. The storage servers use the
expiration times in the storage access requests from the clients in
determining whether to provide or deny the requested access.
FIG. 1 is a functional block diagram of a computing arrangement in
accordance with one embodiment of the invention. Arrangement 100
includes multiple client systems 102a-c, each having a distributed
file system interface 104a-c. File system meta-data is managed by
meta-data server 108, and data server arrangement 116 provides
storage for file data. The components are coupled by a conventional
local or wide area network 118.
When an application hosted on one of clients 102a-c seeks access to
an object stored by data server arrangement 116, the associated DFS
interface 104a-c generates a lease request to meta-data server 108.
The lease request references the object. Depending on
implementation requirements, the lease request optionally includes
a parameter that specifies a lease duration. When the requested
object is available, the meta-data server responds with a lease.
The lease to the client indicates the time at which the lease
commenced and the duration of the lease. Depending on the
implementation requirements, the duration of the lease granted by
the meta-data server may be selected solely at the discretion of
the meta-data server. Alternatively, the duration may be determined
in response to a variety of factors, such as a duration requested
by the client and the number of competing lease requests.
After having been granted a lease to an object, the DFS interface
(on behalf of a client application) prepares to submit a data
access request to the data server arrangement 116. Before
submitting the request, the expiration time of the lease is
determined (lease commencement time+lease duration) using the lease
information returned from the meta-data server 108. The data access
request to the data server arrangement 116 includes a reference to
the data along with the lease expiration time.
Upon receiving a data access request from a client, a storage
server (a component within the data server arrangement 116)
determines whether the requesting client still has a valid lease to
the referenced data using the expiration time in the request. If
the expiration time is before the current time, the lease has
expired and the requesting client is denied access. Otherwise, the
requested access is granted.
In order for the components of the data server arrangement 116 to
determine whether a lease is valid using the expiration time and
current time, the clocks used by the meta-data server and data
server arrangement 116 for lease management must be synchronized.
In one embodiment, synchronization between the components is
performed using a conventional synchronization protocol, for
example, NTP.
In one embodiment, DFS interface 104, meta-data server 108, and
data server arrangement are implemented with conventional hardware
and software adapted to perform the functions described herein.
U.S. patent/application Ser. No. 09/774,841, entitled, "EXTENDING A
STANDARD-BASED REMOTE FILE ACCESS PROTOCOL AND MAINTAINING
COMPATIBILITY WITH A STANDARD PROTOCOL STACK" by Karamanolis et
al., filed on Jan. 31, 2001, and assigned to the assignee of the
present invention, describes another embodiment for implementing
the DFS interface 104a and is hereby incorporated by reference.
In another embodiment, data server arrangement 116 is implemented
with a storage area network (SAN). A SAN can be implemented with an
Internet Protocol (IP) arrangement such as iSCSI, or with a fiber
channel arrangement. Software such as McData's SAN Management and
IBM's Tivoli is required to manage and configure a SAN.
FIG. 2 is an object interaction diagram that illustrates two
clients seeking write access to the same object. The interaction
diagram provides a scenario that shows the potential for error, and
the description below explains how the invention avoids such error
without creating great bandwidth demands or processing demands on
either the meta-data server or storage server. The solid vertical
lines represent control within the corresponding components, and
the directional diagonal lines represent requests and responses
passed between the components.
At time T1, client A submits a write lease request to the meta-data
server for object X. When the meta-data server determines that the
write lease can be granted, the lease is returned to client A at
time T2. According to one embodiment, the lease returned by the
meta-data server includes the time at which the lease commenced and
the duration of the lease. At time T3, client A transmits a write
request to the storage server for block X, which in the scenario
does not arrive at the storage server until time T9. However,
client A's lease expires at time T4.
Before client A's write to block X is received and processed by the
storage server and after client A's lease to block X expires at
time T4, client B submits a write lease request for block X to the
meta-data server at time T5. Since client A's lease expired at time
T4, the meta-data server is able to grant and return the lease to
block X to client B at time T6. At time T7, client B transmits a
write request for block X to the storage server, and at time T8 the
storage server responds with a status code indicating a successful
write.
At time T9, client A's write to block X arrives at the storage
server. If the present invention were not employed, the storage
server would process the write request and overwrite the data in
block X that was written by client B with the data provided by
client A. Furthermore, at time T10 the meta-data server would
respond to client A that the write was successful. If client B's
lease were still valid after time T10 and client B issued a read of
block X to the storage server, the storage server would respond
with that that have been corrupted by client A.
With the present invention, along with each file access request
transmitted to the storage server, the clients transmit expiration
times for the objects referenced in the storage access requests. In
the present example, when client A transmits the write request to
the storage server at time T3, the expiration time, T4, is also
included. When the storage server receives a write request and
lease expiration time, the expiration time is compared to the
current time. If the lease expired before the current time, then
the access is denied. Thus, since client A's lease expired at time
T4, and the storage server received the write request at time T9,
the write request is denied. Assuming that client B's lease
expiration time was sometime after T7, the storage server provides
the write access to object X for client B.
FIG. 3 is a flowchart of a process performed by a client in
submitting a lease request and a storage access request in
accordance with one embodiment of the invention. The "client" in
this flowchart refers to the DFS interface 104a of FIG. 1. At step
302, in response to a file access request from an application
program (not shown), the client generates a lease request. The
lease request includes an object identifier so that the meta-data
server knows which object to lease, along with the type of lease,
read or write, for example. In one embodiment, the lease request
also includes a duration parameter that specifies a requested
duration for the lease. In another embodiment, the duration
parameter is not present, and the meta-data server alone determines
the duration. The lease request is then transmitted to the
meta-data server.
At step 304, the client receives a lease from the meta-data server
and calculates the lease expiration time. In an alternative
embodiment, the meta-data server calculates the lease expiration
time and returns the expiration time in the lease to the client. At
step 306, a storage access request is generated and transmitted to
a storage server responsible for the referenced object. Included in
the storage access request is the lease expiration time for the
referenced object.
At step 308, the client processes the response from the storage
server. For example, for a read request the data provided by the
storage server are returned to the calling application. Success and
failure codes are also processed in accordance with implementation
requirements, and control is returned to the calling
application.
FIG. 4 is a flowchart of a process performed by the meta-data
server in processing a lease request in accordance with one
embodiment of the invention. At step 352, the meta-data server
receives a lease request. The processing performed for a lease
request when the requested object is already leased is
implementation dependent. For example, multiple concurrent read
leases may be permitted. For a write lease request, the request
could be rejected if there are outstanding leases, the request
could be blocked to wait for other leases to expire while rejecting
new lease requests, or the request could trigger a recall of
outstanding leases. Assuming the lease is granted, step 354 marks
the object as leased.
At step 356, the meta-data server generates a lease response for
the client. In one embodiment, the lease includes the time at which
the lease commenced and the duration of the lease. In an
alternative embodiment, the meta-data server calculates the lease
expiration time (start time+duration) and includes the expiration
time in the lease. The method by which the duration is determined
is implementation dependent. For example, the duration may be
determined strictly from a duration requested by the client.
Alternatively, the duration may be determined strictly by the
meta-data server. In yet another embodiment, the duration may be
determined as a function of the duration requested by the client
and the lease activity level associated with the object. After the
duration has been determined, the lease is returned to the
client.
FIG. 5 is a flowchart of a process performed by the storage server
in processing a storage access request in accordance with one
embodiment of the invention. At step 402, the storage server
receives a storage access request. Decision step 404 tests the
validity of the client's lease. If the expiration time in the
storage access request is after the current time, the lease is
valid and the process is directed to step 406. At step 406, the
storage access request is processed in accordance with the type of
access requested (e.g., read or write). A response that indicates
the status of the request is then returned to the client (408).
If the expiration time in the storage access request indicates that
the client's lease has already expired, a rejection status is
returned to the client (410).
Those skilled in the art will recognize other aspects and
embodiments of the present invention from consideration of the
specification and practice of the invention disclosed herein. It is
intended that the specification and illustrated embodiments be
considered as examples only, with a true scope and spirit of the
invention being indicated by the following claims.
* * * * *